The increased use of optical components in communication and data processing systems has created an increased need for a method for measuring optical inhomogeneities in optical components. For example, in fiber optic based communication systems there is a need to measure splice losses and the location of non-reflecting fiber breaks. Similarly, there is a need for a methodology for characterizing optical components such as silica based optical planar wave guides and LiNbO.sub.3 wave guides.
One method for analyzing inhomogeneities in optical fibers is optical time domain reflectometry. In this method, a light pulse is transmitted down the optical fiber and the backscattered light resulting from the interaction of the light pulse with an inhomogeneity in the fiber is measured. The time delay between the incident light pulse and reflected light provides information on the location of the inhomogeneity. The amplitude of the backscatter light signal provides information on the degree of inhomogeneity. In conventional pulsed techniques, the measurement of the backscattered light becomes more difficult as spatial resolution is improved. Higher spatial resolution simultaneously results in lower levels of Rayleigh backscattered light power and increased noise power due to larger receiver bandwidths.
White light interferometry or optical low-coherence reflectometry provides a technique that allows several orders of magnitude improvement in both sensitivity and spatial resolution compared to conventional time domain methods. Spatial resolutions of 20 to 40 microns have been reported using this technique. This resolution is equivalent to the resolution that would be obtained using pulse widths of a few hundred femtoseconds using conventional pulse techniques. For these resolutions, the average Rayleigh backscattered levels for standard telecommunications fibers are of the order of -115 dB. Reflection sensitivities have been limited to values close to the backscattered levels due to the noise intensity of low-coherence optical sources. However, a reflection sensitivity of -136 dB has been demonstrated at a wavelength of 1.3 microns using a high-power superluminescent diode and a balanced detection scheme to eliminate the effects of intensity noise [Takada, et al., "Rayleigh Backscattering Measurements of Single-Mode Fibers by Low Coherence Optical Time-Domain Reflectometry With 14 mm Spatial Resolution", Appl. Phys. Lett., 59, p.143, 1991].
While optical low-coherence reflectometry provides the resolution and sensitivity to perform the measurements in question, the distances over which optical low-coherence reflectometers can scan is limited to about 0.5 meters. This limitation is due to the limited range over which a mirror can be moved using a lead-screw. Hence, inhomogeneities that are separated by more than 0.5 meters cannot be simultaneously detected. There is a need to dramatically increase the scan range because many optical devices that are to be probed have fiber lead lengths greater than one meter. To scan for inhomogeneities separated by larger distances, multiple measurements in the 0.5 meter range must be made with the aid of fixed optical delay lines. This procedure is time consuming.
Broadly, it is an object of the present invention to provide an improved optical low-coherence reflectometry measurement apparatus and method.
It is a further object of the present invention to provide an optical low-coherence reflectometry system that can scan for inhomogeneities separated by many meters.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.